Geoquímica das FFB, chert ferruginosos e rochas vulcânicas do depósito aurífero arqueano de Lamego: modelo deposicional e implicações para metalogênese e contexto geotectônico do greenstone belt Rio das Velhas

Abstract

The paper focuses on metamorphosed banded iron formation and ferruginous chert host gold mineralization in the Lamego orogenic gold deposit, which is inserted in the context of the Neoarchean Rio das Velhas greenstone belt, Quadrilátero Ferrífero. These chemical sedimentary rocks are classified as Algoma-type BIF and occur in a typical association of deep oceanic rocks: intermediate to mafic volcanic, BIF, and carbonaceous pelite. With a relatively low Fe2O3 average content, these rocks are close to the 15% limit that distinguishes BIF and ferruginous chert from one another. Their dominant iron mineral is fine-grained siderite, which is considered the primary phase, darkened by the presence of carbonaceous matter. Ankerite is subordinate, both hydrothermal and primary, whereas magnetite and stilplomelane occur locally and are secondary. Both the BIF and chert have no significant mineralogical and geochemical variations that could have directly influenced gold mineralization, such as facilitating sulfide precipitation. Their geochemistry and rare-earth elements (REE) patterns indicate sedimentation in a deep marine environment with hydrogenous, high temperature hydrothermal fluids and detrital contributions. Among the main characteristics of the PAAS normalized patterns are: positive La, Eu and Ce anomalies; heavy REE enrichment, relative to the light and intermediate REE; chondritic to superchondritic values for Y/Ho ratios. The volcanic rock from the base of the Lamego stratigraphic sequence varies from basalts to andesitic basalts, with a calcium-alkaline signature defined by the ratio La/Sm > 3. This composition is similar to that of the basal volcanic rocks at the nearby, world-class Cuiabá deposit, where these constitute the hosts to mineralized, shear-related quartz veins. The Nb and Ta negative anomalies identified in the multielement spidergram normalized to the primitive mantle indicate that they are closely associated with continental arc basalts. The geochemical results suggest that the Lamego rocks are associated with a marine environment, under the influence of a continental arc, probably related to the Rio das Velhas II event (2800 - 2760 Ma). Alternatively, some authors argue in favor of BIF deposition in restricted basins. In the case of Lamego, the latter hypothesis is supported by the existence of samples with Y/Ho ratios below chondritic values. In addition, these results extend the gold exploration horizon in the Rio das Velhas greenstone, since they further supports the strong epigenetic character of the hydrothermal mineralizing event, which apparently overlapped the entire volcano-sedimentary rock package.